results in an immediate decrease in food and cover for the consumers 

 in the area. The total area affected is usually very small in 

 comparison to the remaining area of productive vegetation because: 

 (1) the gravity meter is quite portable, (2) data stations are not 

 located along predetermined straight lines, and (3) the stations are 

 not regularly spaced. Stations may frequently be accessed by boat 

 or by foot; a maximum of one lightweight surface vehicle is neces- 

 sary for the entire operation. Furthermore, this vehicle need not 

 travel straight-line paths across the marsh surface; it can make 

 maximum use of existing roads or canals. Therefore, the effects on 

 consumers, competition (shading), rough mat formation, and detritus 

 accumulation are negligible. 



The above discussion indicates total vehicular traffic associated 

 with gravity surveys is less than that associated with seismic 

 surveys. Nevertheless, any marsh-surface vehicle may potentially 

 alter local land elevations. Ruts resulting from their use may form 

 depressions for the movement of water and, if deep enough, may 

 result in standing bodies of water during wet seasons. The orienta- 

 tion and depth of the depressions determine whether they (1) remain 

 for long periods of time and (2) alter the water regime of a given 

 area of land by increasing or decreasing the frequency of submergence/ 

 emergence, average depth, and duration of submergence. Areas 

 dominated by marshhay cordgrass may be replaced by stands of three- 

 square or other water-tolerant species when the frequency and/or 

 duration of submergence increases. The converse may occur when 

 frequency and/or duration of submergence decreases. Although the 

 total area affected is site specific, it is usually small when 

 compared to the total area of brackish marsh. Consumers that depend 

 upon these grasses and sedges for food and/or cover will ultimately 

 increase or decrease. 



The depressions allow a faster runoff of surface and standing water 

 in localized areas. The long-term effects on suspended sediments 

 and nutrients are slight. If vehicle tracks occur in areas of the 

 marsh where isolated ponds exist, intramarsh circulation could be 

 affected. Depressions could connect individual water bodies and 

 increase the intermixing of their contents. Waterfowl and wading 

 bird use would be expected to increase in these new depressions with 

 standing water. At the ecosystem level, however, this increase is 

 insignificant. 



The degree and significance of wildlife displacement resulting from 

 grav.ity-meter surveys are slight. This is because of the small 

 number of people, lack of noise, and short period of time required 

 to complete a survey. 



Key attribute alterations 



The potential alteration which could lead to a significant impact on 

 the ecosystem is the creation of depressions in the land surface. 



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